Capturing and separating molecular components from flue gas offers benefits including cost-effective material recycling and energy conservation. Flue gas is an exhaust gas transported by way of pipe or channel from sources such as ovens, fireplaces or furnaces. Separation of flue gas molecular components, such as carbon dioxide gas (CO2) from nitrogen gas (N2), particularly at ambient temperature and pressure, is a crucial factor for capturing carbon. One approach for gas separation involves absorption that uses reactive chemical solvents. Gas absorption is a process in which incorporation of gases into liquid chemical solvents permits separation and recovery of a gas. However, use of chemical solvents for gas separation is expensive, in part, owing to solvent loss from evaporation and corrosion of equipment from the chemical solvent.
Another gas separation approach is adsorption, a process that entails adhesion of molecules (e.g., gas molecules, liquid molecules or solid molecules) to a surface. Desired characteristics of a good adsorbent material include molecule selectivity and reversible loading of molecules onto the adsorbent material. Silica gel and activated carbon are examples of porous materials used for adsorption-based gas separation.
Another example of a porous material used for adsorption-based gas separation is a metal-organic-framework (MOF). MOFs are porous crystalline compounds constructed of building blocks comprising metal ions coordinated to organic ligand molecules. Pore sizes of MOFs can be controlled by appropriate selection of the organic ligand molecules, and desired chemical functionalities can be introduced into the building blocks. However, MOFs also demonstrate limitations for practical use in gas separation. For example, MOFs are often sensitive to air. Moreover, MOFs may possess structural defects, such as incomplete coordination of metals that may prevent efficient gas uptake. The extremely poor solubility of MOFs also makes them impractical for use in membrane technology applications for efficient molecular separations.
Other candidate porous materials for gas separation via adsorption include covalent organic frameworks (COFs) that, in contrast to MOFs, do not contain metal coordination sites. COFs are extended organic structures created by covalently linking COF molecular building blocks. COF building blocks are molecules linked entirely by covalent bonds. Covalent linkage of COF building blocks results in a stable crystalline network structure with directional bonding between the COF building blocks. Advantageously, COFs are light weight, thermally stable, and chemically robust. However, COFs usually have poor solubility and the covalent bonding in rigid, crystalline COF structures limits their usefulness in applications requiring structural flexibility.
Previously, well-defined 3-D frameworks of MOFs and COFs have been achieved through metal-coordination or hydrogen-bonding driven self assembly of 1-D or 2-D building units. These techniques result in thermally unstable and/or chemically unstable and/or insoluble 3-D frameworks that are often difficult to synthesize and exhibit poor selectivity in separating gasses.